US6911764B2 - Energy efficient electroactive polymers and electroactive polymer devices - Google Patents

Abstract

The present invention relates to polymers that convert between electrical and mechanical energy. When a voltage is applied to electrodes contacting an electroactive polymer, the polymer deflects. This deflection may be used to do mechanical work. Similarly, when a previously charged electroactive polymer deflects, the electric field in the material is changed. The change in electric field may be used to produce electrical energy. An active area is a portion of a polymer having sufficient electrostatic force to enable deflection of the portion and/or sufficient deflection to enable a change in electrostatic force or electric field. The present invention relates to energy efficient transducers and devices comprising multiple active areas on one or more electroactive polymers. The invention also relates to methods for actuating one or more active areas on one or more electroactive polymers while maintaining a substantially constant potential energy.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119(e) from co-pending U.S. Provisional Patent Application No. 60/181,404 filed Feb. 9, 2000, which is incorporated by reference for all purposes.

U.S. GOVERNMENT RIGHTS

This application was made in part with government support under contract number N00014-96-C-0026 awarded by the Office of Naval Research. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates generally to electroactive polymers that convert between electrical energy and mechanical energy. More particularly, the present invention relates to energy efficient electroactive polymers and electroactive polymer devices. The present invention also relates to methods of using electroactive polymers and electroactive polymer devices to increase mechanical or electrical output.

In many applications, it is desirable to convert between electrical energy and mechanical energy. Exemplary applications requiring conversion from electrical to mechanical energy include robotics, pumps, speakers, general automation, disk drives and prosthetic devices. These applications include one or more actuators that convert electrical energy into mechanical work—on a macroscopic or microscopic level. Common actuator technologies, such as electromagnetic motors and solenoids, are not suitable for many of these applications, e.g., when the required device size is small (e.g., micro or mesoscale machines) or the weight or complexity must be minimized. Exemplary applications requiring conversion from mechanical to electrical energy include sensors and generators. These applications include one or more transducers that convert mechanical energy into electrical energy. Common electric generator technologies, such as electromagnetic generators, are also not suitable for many of these applications, e.g., when the required device size is small (e.g., in a person's shoe). These transducer technologies are also not ideal when a large number of devices must be integrated into a single structure or under various performance conditions such as when high power density output is required at relatively low frequencies.

Several ‘smart materials’ have been used to convert between electrical and mechanical energy with limited success. These smart materials include piezoelectric ceramics, shape memory alloys and magnetostrictive materials. However, each smart material has a number of limitations that prevent its broad usage. Certain piezoelectric ceramics, such as lead zirconium titanate (PZT), have been used to convert electrical to mechanical energy. While having suitable efficiency for a few applications, these piezoelectric ceramics are typically limited to a strain below about 1.6 percent and are often not suitable for applications requiring greater strains than this. In addition, the high density of these materials often eliminates them from applications requiring low weight. Irradiated polyvinylidene difluoride (PVDF is an electroactive polymer reported to have a strain of up to 4 percent when converting from electrical to mechanical energy. Similar to the piezoelectric ceramics, the PVDF is often not suitable for applications requiring strains greater than 4 percent. Shape memory alloys, such as nitinol, are capable of large strains and force outputs. These shape memory alloys have been limited from broad use by unacceptable energy efficiency, poor response time and prohibitive cost.

In addition to the performance limitations of piezoelectric ceramics and irradiated PVDF, their fabrication often presents a barrier to acceptability. Single crystal piezoelectric ceramics must be grown at high temperatures coupled with a very slow cooling down process. Irradiated PVDF must be exposed to an electron beam for processing. Both these processes are expensive and complex and may limit acceptability of these materials.

In view of the foregoing, alternative devices that convert between electrical and mechanical energy would be desirable.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to polymers that convert between electrical and mechanical energy. When a voltage is applied to electrodes contacting an electroactive polymer, the polymer deflects. This deflection may be used to do mechanical work. Similarly, when a previously charged electroactive polymer deflects, the electric field in the material is changed. The change in electric field may be used to produce electrical energy. An active area is a portion of a polymer having sufficient electrostatic force to enable deflection of the portion and/or sufficient deflection to enable a change in electrostatic force or electric field. The present invention relates to energy efficient transducers and devices comprising multiple active areas on one or more electroactive polymers. The invention also relates to methods for deflecting one or more active areas on one or more electroactive polymers. These methods may result in improved deflection and sometimes improved energy conversion efficiency. In one embodiment, the present invention relates to electroactive polymer transducers and devices that include deflection of an active area that is assisted by mechanical input energy. The mechanical input energy may come from another portion of the polymer, a portion of another polymer, and/or a mechanism coupled to the polymer. In another embodiment, the present invention relates to electroactive polymer transducers and devices that are arranged such that elastic potential energy of the device or transducer is substantially independent of deflection of a first portion of the polymer.

In another aspect, the invention relates to a device for converting between electrical energy and mechanical energy. The device comprises at least one electroactive polymer having a first active area. The first active area comprises at least two first active area electrodes and a first portion of the at least one electroactive polymer. The first portion is arranged in a manner which causes the first portion to deflect in response to a change in electric field provided by the at least two first active area electrodes and/or arranged in a manner which causes a change in electric field in response to deflection of the first portion. The device is arranged such that deflection of the portion in response to a change in electric field and/or deflection of the first portion causing a change in electric field is at least partially assisted by mechanical input energy.

In yet another aspect, the invention relates to a device for converting between electrical energy and mechanical energy. The device comprises at least one electroactive polymer. The at least one electroactive polymer comprises a first active area. The first active area comprising at least two first active area electrodes and a first portion of the at least one electroactive polymer. The first portion is arranged in a manner which causes the first portion to deflect in response to a change in electric field provided by the at least two first active area electrodes and/or arranged in a manner which causes a change in electric field in response to deflection of the first portion. The at least one electroactive polymer is arranged such that elastic potential energy of the device is substantially independent of deflection of the first portion in response to a change in electric field and/or deflection of the first portion causing a change in electric field.

In still another aspect, the invention relates to a method of using at least one electroactive polymer. The at least one electroactive polymer comprises a first active area, the first active area comprising at least two first active area electrodes and a first portion of the at least one electroactive polymer. The method comprising deflecting the first portion such that elastic potential energy of the at least one electroactive polymer is substantially constant for the deflection.

These and other features and advantages of the present invention will be described in the following description of the invention and associated figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a top perspective view of a transducer portion before and after application of a voltage, respectively, in accordance with one embodiment of the present invention.

FIG. 1C illustrates a transducer comprising a plurality of active areas in accordance with one embodiment of the present invention.

FIG. 1D illustrates a device comprising a plurality of symmetrically arranged electrodes in accordance with a specific embodiment of the present invention.

FIG. 2A illustrates a stretched film device in accordance with one embodiment of the present invention.

FIGS. 2B and 2C illustrate a device for converting between electrical energy and mechanical energy in accordance with another embodiment of the present invention.

FIG. 3A demonstrates mechanical input energy and substantially constant elastic energy features using the device ofFIG. 1D in accordance with one embodiment of the present invention.

FIG. 3B illustrates a device comprising two transducers in accordance with one embodiment of the present invention.

FIG. 3C illustrates a device comprising a member that facilitates deflection substantially independent of elastic potential energy in accordance with a specific embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention.

1. Overview

Electroactive polymers convert between mechanical and electrical energy. In one embodiment, the present invention relates to electroactive polymer transducers and devices that comprise deflection of an active area that is assisted by mechanical input energy. The mechanical input energy may come from another portion of the polymer, a portion of another polymer, and/or a mechanical energy input device coupled to the polymer. In another embodiment, the present invention relates to electroactive polymer transducers and devices that are arranged such that elastic potential energy of the device or transducer is substantially independent of deflection of a first portion of the polymer. These transducers and devices may be used in a variety of ways. In a specific embodiment, multiple active areas on a single electroactive polymer may be independently actuated to deflect a portion of the polymer along a two-dimensional path while keeping a substantially constant elastic potential energy. The two-dimensional path may be a circular path used in driving a crank in a motor. Combining different ways to arrange active areas and polymers, different ways to constrain a polymer, scalability of electroactive polymers to both micro and macro levels, and different polymer orientations (e.g., rolling or stacking individual polymer layers) permits a broad range of actuators, motors, sensors and generators that convert between electrical and mechanical energy. These devices find use in a wide range of applications.

For ease of understanding, the present invention is mainly described and shown by focusing on a single direction of energy conversion. More specifically, the present invention focuses on converting electrical energy into mechanical energy, i.e., when a transducer is operating in an actuator. However, in all the figures and discussions for the present invention, it is important to note that the polymers and devices may convert between electrical energy and mechanical energy bi-directionally. Thus, any of the polymer materials, polymer configurations, transducers, and devices described herein are also a transducer for converting mechanical energy to electrical energy (generator mode). Similarly, any of the exemplary electrodes described herein may be used with a generator of the present invention. Typically, a generator of the present invention comprises a polymer arranged in a manner that causes a change in electric field in response to deflection of a portion of the polymer. The change in electric field, along with changes in the polymer dimension in the direction of the field, produces a change in voltage, and hence a change in electrical energy.

For a transducer having a substantially constant thickness, one mechanism for differentiating the performance of the transducer, or a portion of the transducer associated with a single active area, as being an actuator or a generator is in the change in net area orthogonal to the thickness associated with the polymer deflection. For these transducers or active areas, when the deflection causes the net area of the transducer/active area to decrease and there is charge on the electrodes, the transducer/active area is converting from mechanical to electrical energy and acting as a generator. Conversely, when the deflection causes the net area of the transducer/active area to increase and charge is on the electrodes, the transducer/active area is converting electrical to mechanical energy and acting as an actuator. The change in area in both cases corresponds to a reverse change in film thickness, i.e. the thickness contracts when the planar area expands, and the thickness expands when the planar area contracts. Both the change in area and change in thickness affect the amount of energy that is converted between electrical and mechanical. Since the effects due to a change in area and corresponding change in thickness are complementary, only the change in area will be discussed herein for sake of brevity. In addition, although deflection of an electroactive polymer will primarily be discussed as a net increase in area of the polymer when the polymer is being used in an actuator to produce mechanical energy, it is understood that in some cases (i.e. depending on the loading), the net area may decrease to produce mechanical work. Alternatively, when an electroactive polymer is continuously being cycled between actuator and generator modes, electrical or mechanical (elastic) energy may be stored from one part of the cycle for use in other parts of the cycle. This may further introduce situations in which the net area may decrease to produce mechanical work. Thus, devices of the present invention may include both actuator and generator modes, depending on how the polymer is arranged and applied.

2. General Structure of Electroactive Polymers

The transformation between electrical and mechanical energy in devices of the present invention is based on energy conversion of one or more active areas of an electroactive polymer. Electroactive polymers deflect when actuated by electrical energy. To help illustrate the performance of an electroactive polymer in converting electrical energy to mechanical energy,FIG. 1A illustrates a top perspective view of atransducer portion100 in accordance with one embodiment of the present invention. Thetransducer portion100 comprises anelectroactive polymer102 for converting between electrical energy and mechanical energy. In one embodiment, an electroactive polymer refers to a polymer that acts as an insulating dielectric between two electrodes and may deflect upon application of a voltage difference between the two electrodes. Top andbottom electrodes104 and106 are attached to theelectroactive polymer102 on its top and bottom surfaces, respectively, to provide a voltage difference across a portion of thepolymer102.Polymer102 deflects with a change in electric field provided by the top andbottom electrodes104 and106. Deflection of thetransducer portion100 in response to a change in electric field provided by theelectrodes104 and106 is referred to as actuation. Aspolymer102 changes in size, the deflection may be used to produce mechanical work.

FIG. 1B illustrates a top perspective view of thetransducer portion100 including deflection in response to a change in electric field. In general, deflection refers to any displacement, expansion, contraction, torsion, linear or area strain, or any other deformation of a portion of thepolymer102. The change in electric field corresponding to the voltage difference applied to or by theelectrodes104 and106 produces mechanical pressure withinpolymer102. In this case, the unlike electrical charges produced byelectrodes104 and106 attract each other and provide a compressive force betweenelectrodes104 and106 and an expansion force onpolymer102 inplanar directions108 and110, causingpolymer102 to compress betweenelectrodes104 and106 and stretch in theplanar directions108 and110.

In some cases,electrodes104 and106 cover a limited portion ofpolymer102 relative to the total area of the polymer. This may be done to prevent electrical breakdown around the edge ofpolymer102 or to achieve customized deflections for one or more portions of the polymer. As the term is used herein, an active area is defined as a portion of a transducer comprisingpolymer material102 and at least two electrodes. When the active area is used to convert electrical energy to mechanical energy, the active area includes a portion ofpolymer102 having sufficient electrostatic force to enable deflection of the portion. When the active area is used to convert mechanical energy to electrical energy, the active area includes a portion ofpolymer102 having sufficient deflection to enable a change in electrostatic energy. As will be described below, a polymer of the present invention may have multiple active areas. In some cases,polymer102 material outside an active area may act as an external spring force on the active area during deflection. More specifically, polymer material outside the active area may resist active area deflection by its contraction or expansion. Removal of the voltage difference and the induced charge causes the reverse effects.

Electrodes104 and106 are compliant and change shape withpolymer102. The configuration ofpolymer102 andelectrodes104 and106 provides for increasingpolymer102 response with deflection. More specifically, as thetransducer portion100 deflects, compression ofpolymer102 brings the opposite charges ofelectrodes104 and106 closer and the stretching ofpolymer102 separates similar charges in each electrode. In one embodiment, one of theelectrodes104 and106 is ground.

In general, thetransducer portion100 continues to deflect until mechanical forces balance the electrostatic forces driving the deflection. The mechanical forces include elastic restoring forces of thepolymer102 material, the compliance ofelectrodes104 and106, and any external resistance provided by a device and/or load coupled to thetransducer portion100, etc. The deflection of thetransducer portion100 as a result of the applied voltage may also depend on a number of other factors such as thepolymer102 dielectric constant and the size ofpolymer102.

Electroactive polymers in accordance with the present invention are capable of deflection in any direction. After application of the voltage betweenelectrodes104 and106,polymer102 expands (stretches) in bothplanar directions108 and110. In some cases,polymer102 is incompressible, e.g. has a substantially constant volume under stress. For anincompressible polymer102,polymer102 decreases in thickness as a result of the expansion in theplanar directions108 and110. It should be noted that the present invention is not limited to incompressible polymers and deflection of thepolymer102 may not conform to such a simple relationship.

Application of a relatively large voltage difference betweenelectrodes104 and106 on thetransducer portion100 shown inFIG. 1A will causetransducer portion100 to change to a thinner, larger area shape as shown in FIG.1B. In this manner, thetransducer portion100 converts electrical energy to mechanical energy. Thetransducer portion100 may also be used to convert mechanical energy to electrical energy.

FIGS. 1A and 1B may be used to show one manner in which thetransducer portion100 converts mechanical energy to electrical energy. For example, if thetransducer portion100 is mechanically stretched by external forces to a thinner, larger area shapesuch as that shown inFIG. 1B, and a relatively small voltage difference (less than that necessary to actuate the film to the configuration inFIG. 1B) is applied betweenelectrodes104 and106, thetransducer portion100 will contract in area between the electrodes to a shape such as inFIG. 1A when the external forces are removed. Stretching the transducer refers to deflecting the transducer from its original resting position—typically to result in a larger net area between the electrodes, e.g. in the plane defined bydirections108 and110 between the electrodes. The resting position refers to the position of thetransducer portion100 having no external electrical or mechanical input and may comprise any pre-strain in the polymer. Once thetransducer portion100 is stretched, the relatively small voltage difference is provided such that the resulting electrostatic forces are insufficient to balance the elastic restoring forces of the stretch. When the external forces are removed, thetransducer portion100 therefore contracts, and it becomes thicker and has a smaller planar area in the plane defined bydirections108 and110 (orthogonal to the thickness between electrodes). Whenpolymer102 becomes thicker, it separateselectrodes104 and106 and their corresponding unlike charges, thus raising the electrical energy and voltage of the charge. Further, whenelectrodes104 and106 contract to a smaller area, like charges within each electrode compress, also raising the electrical energyand voltage of the charge. Thus, with different charges onelectrodes104 and106, contraction from a shape such as that shown inFIG. 1B to one such as that shown inFIG. 1A raises the electrical energy of the charge. That is, mechanical deflection is being turned into electrical energy and thetransducer portion100 is acting as a generator.

In some cases, thetransducer portion100 may be described electrically as a variable capacitor. The capacitance decreases for the shape change going from that shown inFIG. 1B to that shown in FIG.1A. Typically, the voltage difference betweenelectrodes104 and106 will be raised by contraction. This is normally the case, for example, if additional charge is not added or subtracted fromelectrodes104 and106 during the contraction process. The increase in electrical energy, U, may be illustrated by the formula U=0.5 Q²/C, where Q is the amount of positive charge on the positive electrode and C is the variable capacitance which relates to the intrinsic dielectric properties ofpolymer102 and its geometry. If Q is fixed and C decreases, then the electrical energy U increases. The increase in electrical energy and voltage can be recovered or used in a suitable device or electronic circuit in electrical communication withelectrodes104 and106. In addition, thetransducer portion100 may be mechanically coupled to a mechanical input that deflects the polymer and provides mechanical energy.

Thetransducer portion100 will convert mechanical energy to electrical energy when it contracts. Some or all of the charge and energy can be removed when thetransducer portion100 is fully contracted in the plane defined bydirections108 and110. Alternatively, some or all of the charge and energy can be removed during contraction. If the electric field pressure in the polymer increases and reaches balance with the mechanical elastic restoring forces and external load during contraction, the contraction will stop before full contraction, and no further elastic mechanical energy will be converted to electrical energy. Removing some of the charge and stored electrical energy reduces the electrical field pressure, thereby allowing contraction to continue. Thus, removing some of the charge may further convert mechanical energy to electrical energy. The exact electrical behavior of thetransducer portion100 when operating as a generator depends on any electrical and mechanical loading as well as the intrinsic properties ofpolymer102 andelectrodes104 and106.

In one embodiment,electroactive polymer102 is pre-strained. Pre-strain of a polymer may be described, in one or more directions, as the change in dimension in a direction after pre-straining relative to the dimension in that direction before pre-straining. The pre-strain may comprise elastic deformation ofpolymer102 and be formed, for example, by stretching the polymer in tension and fixing one or more of the edges while stretched. For many polymers, pre-strain improves conversion between electrical and mechanical energy. The improved mechanical response enables greater mechanical work for an electroactive polymer, e.g., larger deflections and actuation pressures. In one embodiment, prestrain improves the dielectric strength of the polymer. In another embodiment, the pre-strain is elastic. After actuation, an elastically pre-strained polymer could, in principle, be unfixed and return to its original state. The pre-strain may be imposed at the boundaries using a rigid frame or may also be implemented locally for a portion of the polymer.

In one embodiment, pre-strain is applied uniformly over a portion ofpolymer102 to produce an isotropic pre-strained polymer. For example, an acrylic elastomeric polymer may be stretched by 200 to 400 percent in both planar directions. In another embodiment, prestrain is applied unequally in different directions for a portion ofpolymer102 to produce an anisotropic pre-strained polymer. In this case,polymer102 may deflect greater in one direction than another when actuated. While not wishing to be bound by theory, it is believed that pre-straining a polymer in one direction may increase the stiffness of the polymer in the pre-strain direction. Correspondingly, the polymer is relatively stiffer in the high pre-strain direction and more compliant in the low pre-strain direction and, upon actuation, more deflection occurs in the low pre-strain direction. In one embodiment, the deflection indirection108 oftransducer portion100 can be enhanced by exploiting large pre-strain in theperpendicular direction110. For example, an acrylic elastomeric polymer used as thetransducer portion100 may be stretched by 100 percent indirection108 and by 500 percent in theperpendicular direction110. The quantity of pre-strain for a polymer may be based on the polymer material and the desired performance of the polymer in an application. Pre-strain suitable for use with the present invention is further described in commonly owned, copending U.S. patent application Ser. No. 09/619,848, which is incorporated by reference for all purposes.

Generally, after the polymer is pre-strained, it may be fixed to one or more objects. Each object is preferably suitably stiff to maintain the level of pre-strain desired in the polymer. The polymer may be fixed to the one or more objects according to any conventional method known in the art such as a chemical adhesive, an adhesive layer or material, mechanical attachment, etc.

Transducers and pre-strained polymers of the present invention are not limited to any particular geometry or type of deflection. For example, the polymer and electrodes may be formed into any geometry or shape including tubes and rolls, stretched polymers attached between multiple rigid structures, stretched polymers attached across a frame of any geometry—including curved or complex geometries, across a frame having one or more joints, etc. Deflection of a transducer according to the present invention includes linear expansion and compression in one or more directions, bending, axial deflection when the polymer is rolled, deflection out of a hole provided on a substrate, etc. Deflection of a transducer may be affected by how the polymer is constrained by a frame or rigid structures attached to the polymer. In one embodiment, a flexible material that is stiffer in elongation than the polymer is attached to one side of a transducer to induce bending when the polymer is actuated.

Materials suitable for use as a pre-strained polymer with the present invention may include any substantially insulating polymer or rubber (or combination thereof) that deforms in response to an electrostatic force or whose deformation results in a change in electric field. One suitable material is NuSil CF19-2186 as provided by NuSil Technology of Carpenteria, Calif. Other exemplary materials suitable for use as a pre-strained polymer include silicone elastomers, acrylic elastomers such as VHB 4910 acrylic elastomer as produced by 3M Corporation of St. Paul, Minn., polyurethanes, thermoplastic elastomers, copolymers comprising PVDF, pressure-sensitive adhesives, fluoroelastomers, polymers comprising silicone and acrylic moieties, and the like. Polymers comprising silicone and acrylic moieties may include copolymers comprising silicone and acrylic moieties, polymer blends comprising a silicone elastomer and an acrylic elastomer, for example. Combinations of some of these materials may also be used as the electroactive polymer in transducers of this invention.

An electroactive polymer layer in transducers of the present invention may have a wide range of thicknesses. In one embodiment, polymer thickness may range between about 1 micrometer and 2 millimeters. Polymer thickness may be reduced by stretching the film in one or both planar directions. In many cases, electroactive polymers of the present invention may be fabricated and implemented as thin films. Thicknesses suitable for these thin films may be below 50 micrometers.

Suitable actuation voltages for electroactive polymers, or portions thereof, may vary based on the material properties of the electroactive polymer, such as the dielectric constant, as well as the dimensions of the polymer, such as the thickness of the polymer film For example, actuation electric fields used to actuatepolymer102 inFIG. 1A may range in magnitude from about 0 V/m to about 440 MV/m. Actuation electric fields in this range may produce a pressure in the range of about 0 Pa to about 10 MPa. In order for the transducer to produce greater forces, the thickness of the polymer layer may be increased. Actuation voltages for a particular polymer may be reduced by increasing the dielectric constant, decreasing the polymer thickness, and decreasing the modulus of elasticity, for example.

As electroactive polymers of the present invention may deflect at high strains, electrodes attached to the polymers should also deflect without compromising mechanical or electrical performance. Generally, electrodes suitable for use with the present invention may be of any shape and material provided that they are able to supply a suitable voltage to, or receive a suitable voltage from, an electroactive polymer. The voltage may be either constant or varying over time. In one embodiment, the electrodes adhere to a surface of the polymer. Electrodes adhering to the polymer are preferably compliant and conform to the changing shape of the polymer. Correspondingly, the present invention may include compliant electrodes that conform to the shape of an electroactive polymer to which they are attached. The electrodes may be only applied to a portion of an electroactive polymer and define an active area according to their geometry. Several examples of electrodes that only cover a portion of an electroactive polymer will be described in further detail below.

Various types of electrodes suitable for use with the present invention are described in commonly owned, copending U.S. patent application Ser. No. 09/619,848, which was previously incorporated by reference above. Electrodes described therein and suitable for use with the present invention include structured electrodes comprising metal traces and charge distribution layers, textured electrodes comprising varying out of plane dimensions, conductive greases such as carbon greases or silver greases, colloidal suspensions, high aspect ratio conductive materials such as carbon fibrils and carbon nanotubes, and mixtures of ionically conductive materials.

Materials used for electrodes of the present invention may vary. Suitable materials used in an electrode may include graphite, carbon black, colloidal suspensions, thin metals including silver and gold, silver filled and carbon filled gels and polymers, and ionically or electrically conductive polymers. In a specific embodiment, an electrode suitable for use with the present invention comprises 80 percent carbon grease and 20 percent carbon black in a silicone rubber binder such as Stockwell RTV60-CON as produced by Stockwell Rubber Co. Inc. of Philadelphia, Pa. The carbon grease is of the type such as NyoGel 756G as provided by Nye Lubricant Inc. of Fairhaven, Mass. The conductive grease may also be mixed with an elastomer, such as silicon elastomer RTV 118 as produced by General Electric of Waterford, N.Y., to provide a gel-like conductive grease.

It is understood that certain electrode materials may work well with particular polymers and may not work as well for others. For example, carbon fibrils work well with acrylic elastomer polymers while not as well with silicone polymers. For most transducers, desirable properties for the compliant electrode may include one or more of the following: low modulus of elasticity, low mechanical damping, low surface resistivity, uniform resistivity, chemical and environmental stability, chemical compatibility with the electroactive polymer, good adherence to the electroactive polymer, and the ability to form smooth surfaces. In some cases, a transducer of the present invention may implement two different types of electrodes, e.g. a different electrode type for each active area or different electrode types on opposing sides of a polymer.

Electronic drivers are typically connected to the electrodes. The voltage provided to electroactive polymer will depend upon specifics of an application. In one embodiment, a transducer of the present invention is driven electrically by modulating an applied voltage about a DC bias voltage. Modulation about a bias voltage allows for improved sensitivity and linearity of the transducer to the applied voltage. For example, a transducer used in an audio application may be driven by a signal of up to 200 to 1000 volts peak to peak on top of a bias voltage ranging from about 750 to 2000 volts DC.

3. Multiple Active Areas

In accordance with the present invention, the term “monolithic” is used herein to refer to electroactive polymers, transducers, and devices comprising a plurality of active areas.

FIG. 1C illustrates amonolithic transducer150 comprising a plurality of active areas in accordance with one embodiment of the present invention. Themonolithic transducer150 converts between electrical energy and mechanical energy. Themonolithic transducer150 comprises anelectroactive polymer151 including twoactive areas152aand152b. Thepolymer151 can be held using, for example, a rigid frame (not shown) attached at the edges of thepolymer151.

Theactive area152ahas top andbottom electrodes154aand154battached to thepolymer151 on its top andbottom surfaces151cand151d, respectively. Theelectrodes154aand154bprovide a voltage difference across aportion151aof thepolymer151. Theportion151adeflects with a change in electric field provided by theelectrodes154aand154b. Theportion151acomprises thepolymer151 between theelectrodes154aand154band any other portions of thepolymer151 having sufficient electrostatic force to enable deflection upon application of voltages using theelectrodes154aand154b. When thedevice150 is used as a generator to convert from electrical energy to mechanical energy, deflection of theportion151acauses a change in electric field in theportion151athat is received as a change in voltage difference by theelectrodes154aand154b.

Theactive area152bhas top andbottom electrodes156aand156battached to thepolymer151 on its top andbottom surfaces151cand151d, respectively. Theelectrodes156aand156bprovide a voltage difference across aportion151bof thepolymer151. Theportion151bdeflects with a change in electric field provided by theelectrodes156aand156b. Theportion151bcomprises thepolymer151 between theelectrodes156aand156band any other portions of thepolymer151 having sufficient stress induced by the electrostatic force to enable deflection upon application of voltages using theelectrodes156aand156b. When thedevice150 is used as a generator to convert from electrical energy to mechanical energy, deflection of theportion151bcauses a change in electric field in theportion151bthat is received as a change in voltage difference by theelectrodes156aand156b.

The active areas for monolithic polymers and transducers of the present invention may be flexibly arranged. In one embodiment, active areas in a polymer are arranged such that elasticity of the active areas is balanced. In another embodiment, a transducer of the present invention includes a plurality of symmetrically arranged active areas.

FIG. 1D illustrates adevice160 comprising a plurality of symmetrically arranged active areas in accordance with a specific embodiment of the present invention. Thedevice160 includes a monolithic transducer comprising four active areas162a-d. Each of the active areas162a-dincludes top and bottom electrodes164a-dattached to apolymer161 on its top and bottom surfaces respectively (only the electrodes164a-don the facing surface of thepolymer161 are illustrated). The electrodes164a-deach provide a voltage difference across a portion of thepolymer161. The electrodes164a-dand their corresponding active areas162a-dare symmetrically and radially arranged around a center point of thecircular polymer161. Correspondingly, the elasticity of the active areas162a-dis balanced.

A firstactive area162ais formed with the two firstactive area electrodes164aand a first portion of theelectroactive polymer161a. Theportion161ais arranged in a manner which causes thefirst portion161ato deflect in response to a change in electric field provided by the firstactive area electrodes164a. Theportion161aincludes thepolymer161 between theelectrodes162aand any other portions of thepolymer161 having sufficient stresses induced by the electrostatic force to enable deflection upon application of voltages using theelectrodes162a. Similarly, a secondactive area162cis formed with the two secondactive area electrodes164cand a second portion of the electroactive polymer161c. The portion161cis arranged in a manner which causes the second portion161cto deflect in response to a change in electric field provided by the at least two secondactive area electrodes164c. A similar arrangement applies to theactive areas162band162d.

A substantiallyrigid frame167 is fixed to the perimeter of thecircular polymer161 such as by using an adhesive. A substantiallyrigid member168 is attached to acentral portion163 of thepolymer161 and allows mechanical output for thedevice160. Themember168 provides mechanical output for thedevice160 based on deflection of thecentral portion163 relative to therigid frame167. Thecentral portion163 is located at least partially between theactive area162aand theactive area162cand at least partially between theactive area162band theactive area162d. Although thecentral portion163 is illustrated as a centrally located circle, it should be understood that thecentral portion163 may be any portion of thepolymer161 between the active areas162a-d. Thus, the substantiallyrigid member168 may be attached to thepolymer161 in any part of thepolymer161 between the active areas162a-dand transfer deflection of that portion as mechanical output of thedevice160.

Actuation of theactive area162amoves thecentral portion163 down. Actuation of theactive area162bmoves thecentral portion163 to the left. Actuation of theactive area162cmoves thecentral portion163 up. Actuation of theactive area162dmoves thecentral portion163 to the right. When electrical energy is removed from theelectrodes164a, thecentral portion163 elastically returns up to its position before actuation of theactive area162a. A similar elastic return occurs for the other active areas164b-d.

The active areas162 are arranged relative to each other such that elastic energy of one active area facilitates deflection of another. Theactive area162ais arranged relative to theactive areas162csuch that elastic energy of theactive area162amay facilitate deflection of theactive area162c. In this case, contraction of theactive area162aat least partially facilitates expansion of theactive area162c, and vice versa. More specifically, deflection of theactive area162aincludes a direction of contraction that is at least partially linearly aligned with a direction of expansion for theactive area162ctowards theactive area162a. In another embodiment, the active areas162a-dare not grouped into pairs. In order for the elastic energy of one active area to facilitate the deflection of another active area, it may only be necessary for the active areas share motion in a common linear direction. In this way the polymer oftransducer160 could have two, three, five or any number of active areas arranged such that the motion of one active area shares a direction with that of another area.

The present invention also includes methods for deflecting one or more electroactive polymers having a plurality of active areas. These comprise include deflection as a result of electrical energy input (actuation) to the polymer and electrical energy output from the polymer (generation). Methods for actuating a transducer or device according to one embodiment of the present invention generally comprise deflecting the first portion such that elastic potential energy of the at least one electroactive polymer is substantially constant for the deflection.

For example, the active areas162a-dmay be actuated sequentially to move thecentral portion163 along acircular path169. As will be described in further detail below, elastic potential energy for thedevice160 is substantially constant along thepath169. To achieve thecircular path169, the active areas162a-dare actuated sequentially in clockwise order and in a timely manner. More specifically, electrical energy is supplied to theelectrodes164bwhile theactive area162acontracts as electrical energy is removed from it. Electrical energy is supplied to theelectrodes164cwhile theactive area162bcontracts. A similar timing is applied in actuating the other active areas to produce thecircular path169. This sequential clockwise actuation may be repeatedly performed to continuously move thecentral portion163 in thecircular path169. Continuous circular output of thecentral portion163 may be used to drive a motor. In a specific embodiment, themember168 may be used as a crank in a rotary crank motor.

Themonolithic transducers150 and160 illustrated and described herein include active areas with similar geometries and symmetrical configurations. It is understood that monolithic polymers of the present invention may include one or more active areas each having a non-symmetrical and custom geometry. It is also understood that active areas on a monolithic polymer may be combined in any configuration. These custom geometry active areas and configurations may be used to produce any custom two-dimensional path or output for a portion of a polymer. For example, the two-dimensional path illustrated above may be achieved with only two active areas without the use of expanding and relaxing pairs as described above. In this case, actuation of one active area and its corresponding elastic return may be used to provide controlled deflection along one linear dimension. Actuation of the other active area and its corresponding elastic return may be used to provide controlled deflection at least partially in an orthogonal linear dimension.

4. Actuator and Generator Devices

The deflection of an electroactive polymer can be used in a variety of ways to produce or receive mechanical energy. Generally, electroactive polymers of the present invention may be implemented with a variety of devices—including conventional actuators and generators retrofitted with an electroactive polymer and custom actuators and generators specially designed for one or more electroactive polymers. Conventional actuators and generators include extenders, bending beams, stacks, diaphragms, etc. Several exemplary devices suitable for use with the present invention will now be discussed. Additional actuators suitable for use with the present invention are described in commonly owned, copending U.S. patent application Ser. No. 09/619,848, which was previously incorporated by reference.

FIG. 2A illustrates a stretchedfilm device270 in accordance with one embodiment of the present invention. The stretchedfilm device270 includes arigid frame271 having ahole272. Anelectroactive polymer273 is attached in tension to theframe271 and spans thehole272. Arigid bar274 is attached to the center of thepolymer273 and provides mechanical output corresponding to deflection of thepolymer273. Therigid bar274 can be different lengths depending on the size of thepolymer273 and the amount of force ormotion279 desired. In one embodiment, therigid bar274 is about 75% of the length of thehole272 along the direction of the central attachment (perpendicular to stroke279).

Compliant electrode pairs275 and276 are patterned on both opposing surfaces of thepolymer273 and on the left and right sides respectively of therigid bar274. When theelectrode pair275 is actuated, a portion of thepolymer273 between, and in the vicinity of, theelectrode pair275 expands relative to the rest of thepolymer273 to move therigid bar274 to the right. Conversely, when theelectrode pair276 is actuated, a second portion of thepolymer273 affected by theelectrode pair276 expands relative to the rest of thepolymer273 and allows therigid bar274 to move to the left. Alternating actuation of theelectrodes275 and276 provides atotal stroke279 for therigid bar274.

One variation of the stretchedfilm device270 includes adding an anisotropic pre-strain to the polymer such that the polymer has high pre-strain (and stiffness) in the direction perpendicular to the rigid bar displacement or a lower prestrain and stiffness in the direction of279. This increases thestroke279. Another variation is to eliminate one of the electrode pairs. For the benefit of simplifying the design, this variation reduces thestroke279 for the stretchedfilm device270. In this case, the portion of the polymer no longer used by the removed electrode now responds passively like a restoring spring.

FIGS. 2B and 2C illustrate adevice300 suitable for use with the present invention. Thedevice300 includes apolymer302 arranged in a manner which causes a portion of the polymer to deflect in response to a change in electric field.Electrodes304 are attached to opposite surfaces of thepolymer302 and cover a substantial portion of the polymer302 (only theforemost electrode304 is illustrated). Twostiff members308 and310 extend alongopposite edges312 and314 of thepolymer302.Flexures316 and318 are situated along the remaining edges of thepolymer302. Eachflexure316 and318 comprises two rigid elements that meet at a joint. The distal ends of each rigid element are attached to thestiff members308 and310. Theflexures316 and318 improve conversion between electrical energy and mechanical energy for thedevice300.

Theflexures316 and318couple polymer302 deflection in one direction into deflection in another direction. In one embodiment, each of the two rigid elements rest at an angle about 90 degrees in the plane of thepolymer302. Upon actuation of thepolymer302, expansion of thepolymer302 in thedirection320 causes thestiff members308 and310 to move apart, as shown in FIG.2C. In addition, expansion of thepolymer302 in thedirection322 causes theflexures316 and318 to straighten, and further separates thestiff members308 and310. In this manner, thedevice300 couples expansion of thepolymer302 in bothplanar directions320 and322 into mechanical output in thedirection320.

One advantage of thedevice300 is that the entire structure is planar. This allows for easy mechanical coupling and simple expansion. For example, thestiff members308 and310 may be mechanically coupled (e.g. glued or similarly fixed) to their respective counterparts of asecond device300 to provide twodevices300 in parallel in order to increase the force output oversingle device300. Alternatively, thestiff member308 from one device may be attached to thestiff member310 from a second device in order to provide multiple devices in series that increase the deflection output over asingle device300.

5. Energy Features

Electroactive polymer material provides a spring force during deflection. Typically, polymer material resists deflection during actuation by its contraction (polymer material outside of an active area) or its expansion (polymer material included in an active area). Removal of the actuation voltage and the induced charge causes the reverse effects. The effects of electroactive polymer elasticity are also witnessed when the polymer is used to convert mechanical energy to electrical energy. In general, when actuation voltages and any external loads are removed, electroactive polymers, or portions thereof, elastically return to their resting position. In one aspect of the present invention, elastic properties of one or more portions of an electroactive polymer, and any energy contribution of external loads, are used to assist electroactive polymer deflection.

In one embodiment, a polymer of the present invention is arranged such that deflection of a portion of the polymer in response to a change in electric field is at least partially assisted by mechanical input energy. As the term is used herein, mechanical input energy refers to mechanical energy that contributes to deflection of a portion of an electroactive polymer. The mechanical input energy provided to a portion of an electroactive polymer may include elastic energy provided by another portion of the electroactive polymer, a portion of another electroactive polymer, a spring, etc. The mechanical input energy may also include energy provided an external load or mechanism coupled to the electroactive polymer. For example, when thedevice160 ofFIG. 1D is used in a crank motor and therigid member168 is a crank arm in the crank motor, a flywheel may be coupled to therigid member168 to assist deflection of one of the active areas162a-din deflecting therigid member168 along thecircular path169.

Cumulatively, the sum of elastic energy in a transducer or device may be referred to as the elastic potential energy of the transducer or device. Elastic potential energy may be used to describe transducers and devices of the present invention and methods of deflecting these transducers and devices. In one embodiment, an electroactive polymer is arranged such that deflection of the electroactive polymer is substantially independent of elastic potential energy. In this case, changes in elastic energy of one or more portions of an electroactive polymer are balanced by the changes in elastic energy in the remainder of the transducer or device. The deflection may be in response to a change in electric field in the polymer and/or deflection of a portion that causes a change in electric field. Since the deflection does not cause a substantial change in the net elastic potential energy of the device, the deflection can be made with very little input energy and force, even though the individual elastic forces internal to the device might be quite large.

FIG. 3A demonstrates mechanical input energy and substantially constant elastic energy deflection using thedevice160 ofFIG. 1D in accordance with one embodiment of the present invention. Thedevice160 includes four substantially equipotential lines180a-d. Theequipotential line180acorresponds to thecircular path169 of FIG.1D. The active areas162a-dare able to move thecentral portion163 along any one of the equipotential lines180a-d. When the active areas162 move thecentral portion163 along any one of the equipotential lines180a-d, elastic potential energy of thedevice160 is substantially independent of the position of thecentral portion163 on the line. In other words, the elastic potential energy of thedevice160 remains substantially constant as thecentral portion163 moves along any one of the equipotential lines180a-d.

As a result of this elastic energy balance, electrical input used for actuation of thedevice160 does not need to overcome elastic energy of thepolymer161 as thecentral portion163 moves along any one of the equipotential lines180a-d. Actuating thedevice160 in this manner results in an increased mechanical output for a given electrical input compared to actuation of thedevice160 when not using one of the equipotential lines180a-d. The increased mechanical output may be used to increase deflection of theportion163.

In one embodiment, a substantially loss-less motion constraint or mechanism, such as a crank or a roller on a round surface, constrains the deflection of thedevice160 and holds therigid member168 along a given equipotential line180a-d. The rigid motion constraint or mechanism provides the necessary forces perpendicular to a given equipotential line180a-dat any given point to offset the elastic forces in that direction. In another embodiment, actuation may occur without a motion constraint device or constraint forces along a constant energy path. In this case, thedevice160 may need to supply energy to keep on a given equipotential line180a-dand may reduce efficacy along one of the equipotential lines180a-d.

Deflection of thedevice160 includes mechanical input energy from different portions of thepolymer161. The mechanical input energy includes elastic energy contributions provided by contractions and expansions of each of the active areas162 and portions of thepolymer161 outside the active areas162. A motion constraint such as a crank does not provide any mechanical input energy by themselves, but it provides mechanical forces perpendicular to motion on an equipotential line to assist the actuation by holding the motion to a path of constant elastic energy, and thereby eliminate the need for the expansion and contraction of the polymer to provide these forces. The amount of mechanical input energy and timing of actuation may vary. In one embodiment, the total mechanical input energy provided by different portions of thepolymer161 is substantially equal to the elastic energy required to deflect the firstactive area162afor a part of the deflection. In another embodiment, the total mechanical input energy provided by different portions of thepolymer161 is substantially equal to the elastic energy required to deflect the firstactive area162afor an entire deflection corresponding to an actuation of one of the active areas162.

For deflection along any one of the equipotential lines180a-d, the total elastic energy for stretching portions of thepolymer161 during actuation of one or more of the active areas162a-dis substantially equal to the total elastic energy of contracting portions of thepolymer161. With the elastic energy balanced between the different portions of thepolymer161 along any one of the equipotential lines180a-d, the mechanical output energy for thedevice160 is greater for a given input voltage compared to an arrangement where the elastic energy is not balanced and deflection crosses two or more equipotential lines180a-d. In other words, deflection that crosses equipotential lines restricts the deflection due to imbalanced elastic forces. In contrast, deflection along one of the equipotential lines180a-dincreases deflection and mechanical output energy for a given input voltage.

Therigid member168 provides mechanical output for thedevice160. To increase mechanical output using of thedevice160, the input energy methods ofFIG. 3A may be applied to therigid member168. More specifically, therigid member168 may include a direction of output motion that is at least partially along one of the equipotential lines180a-d. In addition, an external load (e.g., the flywheel described above) coupled to therigid member168 may also assist the rigid member in following one of the equipotential lines180a-d. For example,rigid member168 may be constrained to follow an equipotential line180a-dby attaching it to a crank which constrains the motion ofrigid member168 to a circular path that corresponds to one of the equipotential lines180a-d.

An active area may include multiple directions of contraction and expansion. Correspondingly, elastic energy generated during actuation of one active area may used to facilitate deflection of more than one other active area. For thedevice160, the active areas162 are arranged relative to each other such that elastic return of one active area162a-dmay facilitate deflection of more than one other active area162a-din a direction of actuation. More specifically, theactive areas162aand162care arranged such that contraction of theactive area162amay facilitate expansion of theactive area162cin a direction towards theactive area162a. In addition, theactive areas162aand162bare arranged such that contraction of theactive area162amay facilitate expansion of theactive area162bin a direction towards theactive area162a.

The timing of deflection between active areas may affect elastic energy transfer therebetween. To increase elastic energy transfer for thetransducer160, theactive areas161a-dmay be actuated at a high enough rate such that elastic return of one active area assists the deflection of more than one active area subsequently actuated. This may be useful for active areas having more than one direction of actuation. For example, to increase elastic energy transfer to theactive areas162band161c, actuation ofactive areas162band161cmay begin actuation during elastic return ofactive area161a. In this manner, elastic energy generated during actuation ofactive area162ais transferred to twoactive areas162band162cactuated thereafter. A similar timing may be continuously applied as the active areas162a-dare actuated in turn.

For thedevice160, there is a complementary nature of the active areas162a-don opposite sides of therigid member168. It should be noted that active areas for a device need not be grouped in complementary pairs as described with thedevice160. For example, an odd number of active areas arranged around therigid member168 may still employ the elastic energy balance and mechanical input energy features described above. More specifically, three active areas arranged around therigid member168 at 120 degree intervals may still employ the elastic energy balance and mechanical input energy features described above. In this case, the expansion of one active area is paired with the contraction of more than one other active area.

The active areas for a polymer may be arranged in different ways depending on an application. As mentioned before, it is understood that a monolithic polymer of the present invention may include one or more custom geometry active areas. The arrangement and number of these custom geometry active areas may affect elastic energy transfer and the shape of any equipotential lines.

Any electroactive polymer device with more than one active area, element, or passive element that balances the elastic energy of the active element will have one or more equipotential lines. As one of skill in the art will appreciate, a device may include an infinite number of equipotential line separated by small increments. In many cases, the equipotential lines will not correspond to a simple geometric path as described above and may have complex shapes. Equipotential lines for a transducer or device may be estimated by making physical assumptions about the transducers. For example, the transducers may be assumed to behave elastically like springs based on known elastic moduli of a polymer and the polymer geometry. Alternatively, equipotential lines for a polymer or device may be estimated by directly measuring the mechanical energy needed to deflect the polymer or device. For example, the elastic energy behavior of an individual transducer during stretching and contracting may be measured to produce an energy-displacement curve. The energy-displacement curve may then be used to estimate equipotential paths of a collection of identical transducers that are similarly configured.

Generally, the elastic energy features of the present invention are applicable to any combination of active areas, polymers, transducers and/or devices, such that the total elastic potential energy of the device is substantially independent of deflection. For example, the elastic energy methods of the present invention apply to a plurality of active areas on different polymers and devices. In one embodiment, a device of the present invention comprises a first electroactive polymer including the first active area and a second electroactive polymer including the second active area.

In another embodiment, electroactive polymers in a device are arranged such that elasticity of the polymers is balanced. Further, the active areas on the multiple polymers may be arranged such that their areas are elastically balanced. In yet another embodiment, a plurality of active areas are symmetrically arranged on multiple polymers of a device. Advantageously, transferring elastic energy between polymers may eliminate the need for electroactive forces generated by electrodes to overcome some of the elastic resistance of one of the polymers. In one embodiment, the mechanical input energy provided to a portion of a polymer is less than the elastic energy required to deflect the first portion of the electroactive polymer for a part of the deflection. In some cases, elastic energy may be transferred between polymers without external assistance. In other cases, one or more external mechanisms may be used to transfer elastic energy of one polymer to another. The mechanisms may include cables, belts, pulleys, levers, etc.

FIG. 3B illustrates a device191 comprising twotransducers192aand192bin accordance with one embodiment of the present invention. Thetransducers192aand192bare each rotatably pinned at theirproximate ends193aand193b, respectively. Thetransducers192aand192bare also rotatably pinned at theirdistal ends194aand194b, respectively, using a pinnedconnection195.

Thetransducers192aand192bare able to move the pinnedconnection195 along any one of a set of equipotential lines194a-f. Motion along one of the equipotential lines194a-fuses a suitable motion constraint such as an ellipse-shaped surface with a roller connected to pin195. The motion constraint constrains thepin195 to move along one of the equipotential lines194a-f. When thetransducers192 move theconnection195 along one of the equipotential lines194a-f, elastic potential energy of the device191 is substantially independent of the position of theconnection195. In other words, the elastic potential energy of the device191 remains substantially constant as theconnection195 moves along any one of the equipotential lines194a-f. In this case, elastic potential energy of the lines194a-fincreases with distance away from geometricallycentral point196. Thecentral point196 lies at the half point of the linear distance between the pinned proximate ends. During operation in which the actuator output operates along an equipotential line, the motion constraint,point196 will not usually be reached.

Thetransducers192aand192bare arranged relative to each other such that elastic energy of one transducer assists deflection of the other. In this case, contraction of thetransducer192amay assist expansion of thetransducer192b, and vice versa. More specifically, deflection of thetransducer192aincludes a direction ofcontraction197 that is at least partially linearly aligned with a direction of expansion for thetransducer192b. The input energy transferred from thetransducer192ato thetransducer192bis generated in thetransducer192aduring actuation of thetransducer192a. By transferring elastic energy in this manner, elastic potential energy from a first polymer may be used to at least partially overcome elastic strain work required in actuating a second polymer.

As a result of the energy transfer between thetransducers192aand192b, the amount of electrical energy required to continually actuate thetransducers192 to move theconnection195 along one of the equipotential lines194a-fwith a motion constraint is reduced. This reduction in energy may lead to less electrical energy supplied to electrodes in communication with thetransducers192. In other words, the input electrical energy to thetransducers192 will be equal to the amount of energy required to deflect theconnection195 along one of the equipotential lines194a-fover and above the amount of elastic energy saved by deflecting theconnection195 along one of the equipotential lines194a-f. Collectively, the total amount of input electrical energy required for continuous actuation of thetransducers192 is reduced by the continuous elastic energy transferred betweentransducers192 as theconnection195 follows one of the equipotential lines194a-f. Advantageously, when devices of the present invention are arranged and actuated in this manner, the reduced amount of electrical energy required for actuation may result in improved electrical to mechanical efficiency. Alternatively, the same amount of input electrical energy may result in an increased mechanical output.

Inherently, any electroactive polymer device may include one or more equipotential lines. To help fit an equipotential line to a desired deflection, external loads having known elasticities may be used to manipulate the elastic energy of a device. For example, a rubber band or spring may be coupled to a polymer to change the elastic energy for deflection in a specific direction. In another embodiment, devices of the present invention may include members or structures that facilitate independent elastic potential energy deflection. For example, theconnection195 inFIG. 3B may be attached to a cam system including a roller or wheel that is constrained to move on a solid surface in the shape of one of the equipotential lines194a-f.

FIG. 3C illustrates adevice184 comprising a member that facilitates substantially independent elastic potential energy deflection in accordance with a specific embodiment of the present invention. Thedevice184 includes twolinear output transducers186aand186b. Thelinear output transducers186aand186bproduce deflection in a vertical direction185a. In one embodiment, thetransducers186aand186bare each included in an actuator that facilitates linear deflection in the vertical direction185a, e.g., thedevice300 of FIG.2B. Thetransducers186aand186bare made from the same material and provide a substantially similar spring force for a given deflection in the vertical direction185a.

Each of thetransducers186aand186bis mechanically coupled to alever188 usingcables187aand187b, respectively. Thelever188 rotates around apivot point189. Thelever188 includessides188aand188bhaving substantially the same length and extending from a right angle included in thelever188 at thepivot point189. For purposes of illustration, anangle190 is defined between either of thesides188aand188band aline185bperpendicular to the vertical direction185aand located at thepivot point189. When the length of the cables is large compared to the displacement of thetransducers186aand186b, andtransducers186aand186bare initially attached with zero deflection whenangle190 is 0 and 90 degrees, respectively, the elastic potential energy of thedevice184 is substantially independent oflever188 deflection about thepivot point189.

For elastic deflection of most polymers, a larger deflection presents an increase in force and elastic energy. Thelever188 facilitates substantially independent elastic potential energy deflection for thedevice184. Thelever188 does so by producing a total potential energy for thedevice184 that is substantially constant, regardless of deflection about thepivot point189 and deflection for thetransducers186aand186b. The total potential energy for thedevice184 may be calculated from the geometry of the lever188 (based on the length of thesides188aand188b, deflection relative to theline185band deflection of each of thetransducers186aand186b). When the polymers act elastically for the displacements of interest, the total potential energy does not substantially change for anyangle 190 and deflection of thetransducers186aand186b. Thus, thelever188 allows differences in deflection of thetransducers186aand186b, and differences in forces and elastic energy resulting therefrom, to produce a substantially constant total potential energy for thedevice184.

In many cases, it is desirable for an electroactive polymer device return to a specific or “home” position when it is turned off. The home position has a lower elastic potential energy than the substantially constant elastic potential energy of thedevice184 during deflection. In one embodiment, the home position is acheived by designing thedevice184 such that it does not follow an equipotential path precisely, but rather so that it has a minimum elastic energy at one point (the home position) and elastic energy of the device decreases in the direction from any point to the home position. Thus, when electrical energy is removed from the device, it deterministically returns to the home position via decreasing elastic energy. The restoring force driving the device toward the home position may be increased by increasing the deviation from the equipotential path or configuration.

Using thedevice184 ofFIG. 3C as an example, thedevice184 is substantially equipotential with thetransducers186aand186brelatively far away from thepivot point189 and thesides188aand188bbeing at a right angle to each other. If the angle included in thelever188 betweensides188aand188bis increased beyond 90 degrees for one of thetransducers186aand186b, thelever188 surface is not equipotential and the lowest energy state occurs when thetransducers186aand186bare of equal length (whencables187aand187bare of equal length). In this case, when the voltage applied to thetransducers186aand186bis set to zero, thedevice184 will automatically return to the configuration where thetransducers186aand186bhave equal length—assuming there is no applied external forces or torques on thelever188. The force or torque driving thedevice184 to the home position may be increased by increasing the lever angle further beyond 90 degrees. This however, may reduce the ability to move on an equipotential path.

In one embodiment, actuation of a second active area begins when a first active area is at peak deflection. In another embodiment, actuation of a second active area begins after application of a voltage difference to the at least two first active area electrodes ends. In yet another embodiment, an active area is actuated in resonant mode. Operating an electroactive polymer at resonance using materials, such as silicone, with low losses (e.g., low viscoelastic losses) allows energy available from the elastic return to stay in the polymer in the form of resonant mode vibration or kinetic energy for use in a subsequent actuation.

Although the elastic energy features of the present invention have been discussed with respect to transducers comprising multiple active areas and devices comprising multiple transducers, some of the elastic energy features of the present invention also apply to devices and transducers comprising a single active area. For example, resonant mode actuation also works well with transducers and devices that include a single active area on a single polymer. In this case, energy available from the elastic return of the active area may stay in the polymer in the form of resonant mode vibration and used by the same active area.

6. Applications

The devices and methods of the present invention finds use in a broad range of applications where conversion between electrical and mechanical energy is required. These applications include a wide variety of actuators, motors, generators, sensors, robotics, toys, micro-actuator applications and pumps. Transducers of the present invention may be implemented in both the micro and macro scales—thus increasing the range of application. Provided below are several exemplary applications for some of the transducers and devices described above. The exemplary applications described herein are not intended to limit the scope of the present invention. As one skilled in the art will appreciate, the transducers of the present invention may find use in any application requiring conversion between electrical and mechanical energy.

By repeatedly actuating an electroactive polymer, continuous deflection of the polymer may produce reciprocating linear motion or continuous rotary motion. Reciprocating linear motion may be converted to continuous rotary motion using clutches, gears and the like. Continuous rotary motion generated by an electroactive polymer may be used to drive a motor. Combining different ways to configure one or more electroactive polymers within a motor, different motor designs, scalability of electroactive polymers to both micro and macro levels, and different polymer orientations (e.g., rolling or stacking individual polymer layers) permits a broad range of motor designs comprising one or more electroactive polymers as described herein. These motors convert electrical energy into mechanical work and find use in a wide range of applications. As one of skill in the art will appreciate, there are numerous applications for motors. Due to the weight savings gained by using electroactive polymers in producing mechanical energy for a motor, a motor comprising an electroactive polymer is well suited for motor applications that require a lightweight motor. For example, the present invention is well suited for applications that require a lightweight motor that can operate at low speeds and yet obtain high-performance from the electroactive polymer materials. There are many applications for a lightweight, low rpm, efficient motor.

The present invention is also suitable for use as artificial muscle. In one example of artificial muscle, a device comprises two or more layers of electroactive polymer are sandwiched together and attached to two rigid plates at opposite edges of each polymer. Electrodes are sealed into the center between each of the polymer layers. Each of the polymer layers may include one or more active areas. An advantage of the layered construction is that multiple electroactive polymer layers may be stacked in parallel in order to produce a desired force that would otherwise not obtainable using a single polymer layer. In addition, the stroke of a linear device may be increased by adding similar linear motion devices in series.

In another embodiment, electroactive polymers suitable for use with the present invention may be rolled or folded into linear transducers and devices that deflect axially while converting between electrical energy and mechanical energy. Since the fabrication of monolithic electroactive polymers is often simpler with fewer numbers of layers, rolled actuators provide an efficient manner of fitting a large number of polymer layers into a compact shape. Rolled or folded transducers and devices typically include two or more layers of polymer. Rolled or folded actuators are applicable wherever linear actuators are used, such as robotic legs and fingers, high force grippers, etc.

Polymers comprising one or more active areas that are rolled into a tubular or multilayer cylinder actuator may be implemented as a piston that expands axially upon actuation. Such an actuator is analogous to a hydraulic or pneumatic piston, and may be implemented in any device or application that uses these traditional forms of linear deflection. An electroactive polymer comprising multiple active areas may also operate at high speeds for a variety of applications including sound generators and acoustic speakers, inkjet printers, fast MEMS switches etc.

8. Conclusion

While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents that fall within the scope of this invention which have been omitted for brevity's sake. For example, although the present invention has been described in terms of several specific electrode materials, the present invention is not limited to these materials and in some cases may include air as an electrode. In addition, although the present invention has been described in terms of several preferred polymer materials and geometries, the present invention is not limited to these materials and geometries. It is therefore intended that the scope of the invention should be determined with reference to the appended claims.

Claims (35)

1. A device for converting between electrical energy and mechanical energy, the device comprising

an electroactive polymer having a first active area, the first active area comprising

at least two first active area electrodes and

a first portion of the electroactive polymer, the first portion arranged in a manner which causes the first portion to deform in response to a change in electric field provided by the at least two first active area electrodes and/or arranged in a manner which causes a change in electric field in response to deflection of the first portion,

a second portion of the electroactive polymer that stores mechanical input energy in tension,

wherein the device is arranged such that elastic potential energy of the polymer is substantially independent of deformation of the first portion in response to a change in electric field and/or deformation of the first portion causing a change in electric field.

2. The device ofclaim 1 wherein the mechanical input energy is substantially equal to the elastic energy required to deflect the first portion of the electroactive polymer for a part of the deformation.

3. The device ofclaim 2 wherein the mechanical input energy is substantially equal to the elastic energy required to deflect the first portion of the electroactive polymer for an entire deformation corresponding to an actuation.

4. The device ofclaim 1 wherein the mechanical input energy is less than the elastic energy required to deflect the first portion of the electroactive polymer for a part of the deformation.

5. The device ofclaim 1 wherein the first portion and the second portion are arranged such that deformation of the first portion includes a direction of expansion that is at least partially linearly aligned with a direction of contraction for the second portion.

6. The device ofclaim 1 wherein the device is arranged such that elastic potential energy of the polymer remains substantially constant during deformation of the first portion.

7. The device ofclaim 5 further comprising a second active area, the second active area comprising at least two second active area electrodes and the second portion of the electroactive polymer, the second portion arranged in a manner which causes the second portion to deform in response to a change in electric field provided by the at least two second active area electrodes and/or arranged in a manner which causes a change in electric field in response to deformation of the second portion.

8. The device ofclaim 7 wherein elastic potential energy stored in the second portion during actuation of the second active area at least partially contributes to the mechanical input energy.

9. The device ofclaim 7 wherein the first active area and the second active area are arranged such that deformation of the first portion comprises a direction of contraction that is at least partially linearly aligned with a direction of expansion for the second portion.

10. The device ofclaim 7 wherein the electroactive polymer is a monolithic polymer.

11. The device ofclaim 10 wherein the monolithic polymer has a substantially symmetrical geometry.

12. The device ofclaim 10 wherein the first active area and the second active area are arranged substantially symmetrically about a central portion of the monolithic polymer.

13. The device ofclaim 1 wherein the electroactive polymer is arranged such that elastic potential energy of the electroactive polymer is independent of deformation in response to a change in electric field provided by the at least two first active area electrodes and/or deformation which causes a change in electric field.

14. The device ofclaim 1 further comprising one or more structures that constrain deformation of the polymer to move along an equipotential line of substantially constant elastic potential energy.

15. The device ofclaim 14 wherein the mechanical input energy is provided by an external load coupled to the one or more constraining structures.

16. The device ofclaim 1 wherein the device is included in one of an actuator, a motor and a generator.

17. A device for converting between electrical energy and mechanical energy, the device comprising

at least one electroactive polymer, the at least one electroactive polymer comprising

a first active area, the first active area comprising at least two first active area electrodes and

a first portion of the at least one electroactive polymer, the first portion arranged in a manner which causes the first portion to change in response to a change in electric field provided by the at least two first active area electrodes and/or arranged in a manner which causes a change in electric field in response to deformation of the first portion,

one or more structures that constrain deformation of the polymer such that elastic potential energy of the device, which is the sum of the elastic energy in the device, is substantially independent of deformation of the first portion in response to a change in electric field and/or deformation of the first portion causing a change in electric field.

18. The device ofclaim 17 wherein the at least one electroactive polymer is arranged such that elastic potential energy of the device is substantially constant during deformation of the first portion in response to a change in electric field and/or deformation of the first portion causing a change in electric field.

19. The device ofclaim 18 further comprising a home position having a lower elastic potential energy than the substantially constant elastic potential energy of the device during deformation of the first portion.

20. The device ofclaim 17 further comprising a second active area, the second active area comprising at least two second active area electrodes and a second portion of the at least one electroactive polymer, the second portion arranged in a manner which causes the second portion to deflect in response to a change in electric field provided by the at least two second active area electrodes and/or arranged in a manner which causes a change in electric field in response to deformation of the second portion.

21. The device ofclaim 20 wherein the first active area and the second active area are arranged such that deformation of the first portion includes a direction of contraction that is at least partially linearly aligned with a direction of expansion for the second portion.

22. The device ofclaim 21 wherein the at least one electroactive polymer is a monolithic polymer that comprises both the first portion and the second portion.

23. The device ofclaim 20 wherein the first portion of the at least one electroactive polymer is included in a first electroactive polymer that is mechanically coupled to a second electroactive polymer that comprises the second portion.

24. The device ofclaim 17 wherein the one or more structures constrain deformation of the polymer to move along an equipotential line of substantially constant elastic potential energy.

25. The device ofclaim 24 wherein the one or more structures provide forces perpendicular to the equipotential line during the deformation.

26. A method of using at least one electroactive polymer, the at least one electroactive polymer comprising a first active area, the first active area comprising at least two first active area electrodes and a first portion of the at least one electroactive polymer, the method comprising deflecting the first portion such that elastic potential energy of the at least one electroactive polymer is substantially constant for the deformation.

27. The method ofclaim 26 wherein the deformation is in response to a change in electric field provided by the at least two first active area electrodes.

28. The method ofclaim 26 further comprising deflecting a second portion of the at least one electroactive polymer, the second portion included in a second active area having at least two second active area electrodes.

29. The method ofclaim 28 wherein deflecting of the second portion is at least partially assisted by elastic energy stored in the first portion.

30. The method ofclaim 28 wherein application of a voltage difference to the at least two second active area electrodes begins after application of a voltage difference to the at least two first active area electrodes ends.

31. The method ofclaim 28 wherein application of a voltage difference to the at least two second active area electrodes begins while the first portion of the at least one electroactive polymer is contracting.

32. The method ofclaim 31 wherein application of a voltage difference to the at least two second active area electrodes begins during the peak elastic contraction of the first portion of the at least one electroactive polymer.

33. The method ofclaim 28 wherein actuating the second active area includes a direction of expansion that is at least partially linearly aligned with a direction of contraction of the first active area after actuation.

34. The method ofclaim 28 wherein the first portion and second portion are deformed to move a third portion of the at least one electroactive polymer along a path.

35. The method ofclaim 26 wherein the first portion is deformed in resonant mode.

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